The invention relates to a method for determining the performance of a storage battery by evaluating the time profile of the voltage drop with a heavy current load.
For most applications, the performance of a storage battery is measured by its capability to emit or absorb a specific amount of electrical power without predetermined voltage thresholds being undershot or overshot in the process. One major influencing variable for this capability is the internal resistance of the starter battery at that time, which is governed firstly by the type of battery and secondly, in general, both by the battery temperature TBAT at that time and the state of charge SOC of the battery at that time, as well as the wear that has already occurred.
To determine the internal resistance, the battery voltage is normally measured at different load currents, and the resistance is calculated from the numerical ratio of the voltage change to the current change.
The validity of such a method becomes better the greater the current change, and the greater the voltage change associated with it. Such methods require test equipment to measure the voltage and current over a wide value range. However, it is often desirable to avoid using additional costly test equipment to measure the battery current.
DE 3901680-C1 discloses a method for monitoring the cold-starting capability of the starter battery for an internal combustion engine, in which the time profile of the voltage drop that occurs during starting is monitored and evaluated. The evaluation is in this case carried out on the basis of limit values of an empirically obtained characteristic, and as a function of the battery temperature.
DE 3712629-C1 discloses a measurement apparatus for the remaining life of a motor vehicle battery, in which the battery voltage and the associated load current value are detected before and after initial starting with the battery in the fully charged state, the temperature-compensated internal resistance is determined and stored in a memory, and compared with the internal resistance values determined during the subsequent starting processes for the internal combustion engine. The indication is then produced as a function of predetermined, stored threshold values.
DE 197 50 309 A1 describes a method for determining the starting capability of the starter battery for a motor vehicle, in which the voltage drop during starting is measured and compared with the values in a family of characteristics. This family of characteristics comprises the mean value of the voltage drops measured during the starting process and the associated battery and engine temperatures. If the difference exceeds a predetermined value, then an indication or alarm function is triggered. The measured values of the voltage drop taken into account in the family of characteristics originate, in one particular embodiment, from the new phase period, which lasts for 6 to 18 months, in the sense of a learning phase for the normal state, before any wear occurs, and thus takes account of the respective characteristics in the vehicle, such as characteristics of the installed battery, the internal combustion engine and of the electrical installation.
Thus, it would be advantageous to provide an improved method for assessment of the present performance of a storage battery, which also allows prediction of the performance of the battery in other states, for example, a different state of charge SOC and a different battery temperature TBAT, than the present states.
This invention relates to a method for determining performance of a storage battery including evaluating a time profile of a voltage drop in the storage battery by application of a heavy current load, determining a voltage A from voltage response U(t) of the storage battery after switching on the heavy current load, determining a state value A1 from the voltage value A, battery temperature TBAT and state of charge SOC, comparing state value A1 with a preset value A1x which depends at least on associated battery temperature (TBAT) and associated state of charge (SOC) of the storage battery, wherein the present value A1x is calculated from comparison values A1T which are determined from the state values A1 for previous heavy current loads applied to the storage battery, and determining performance of the storage battery from the difference between the present value A1xand the state value A1.